A Lower Bound on the Mass of Cold Thermal Dark Matter from Planck

TL;DR

This paper uses Planck satellite measurements of neutrino numbers to establish a lower mass limit of about 3.5 MeV for cold thermal dark matter that interacts with standard particles, regardless of its annihilation process.

Contribution

It provides a robust lower bound on cold thermal dark matter mass based on cosmological data, applicable to various interaction scenarios and annihilation channels.

Findings

Lower bound of 3.5 MeV on neutralino dark matter mass.

Constraints apply to dark matter in thermal equilibrium with electrons, photons, or neutrinos.

Limits are independent of s-wave or p-wave annihilation cross-sections.

Abstract

We show that the new measurement of the effective number of neutrinos Neff by the Planck satellite can be used to set a robust lower bound on the mass of cold thermal dark matter of O(MeV). Our limit applies if the dark matter remains in thermal equilibrium by coupling to electrons and photons or through interactions with neutrinos, and applies regardless of whether the dark matter annihilation cross-section is s-wave or p-wave. To illustrate our bounds we apply them to a model of a supersymmetric neutralino annihilating to neutrinos, via a light mixed left-right handed sneutrino mediator. While this scenario was not constrained by previous data, the Planck limits on Neff allow us to set a lower bound on the neutralino dark matter mass of 3.5 MeV.